Co-photolysis of two simple N-heterocyclic phosphenium complexes with H2 proceeds in one case under cooperative addition of H2 across the P=Mn double bond and in the other case via decarbonylation without participation of H2. The origin of this divergence and preliminary results on the passing on of the H2 molecule to styrene are discussed.
Abstract
The reactions of two complexes [(RNHP)Mn(CO)4] (RNHP=N-arylated N-heterocyclic phosphenium) with H2 at elevated pressure (≈4 bar) were studied by NMR spectroscopy. Irradiation with UV light initialized in one case (5 a, R=Dipp) the unselective formation of (RNHP-H)MnH(CO)4] (6 a) via cooperative addition of H2 across the Mn=P double bond. In the other case (5 b, R=Mes), addition of H2 was unobservable and the reaction proceeded via decarbonylation to a dimeric species [(RNHP)2Mn2(CO)7] (7 b) that was isolated and identified spectroscopically. Taking into account the outcome of further reaction studies under various conditions in the absence and presence of H2, both transformations can be explained in the context of a common mechanism involving decarbonylation to 7 a,b as the first step, and the different outcome is attributable to the fact that 7 b is unreactive towards both H2 and CO while 7 a is not. DFT studies relate this divergence to deviations in the molecular constitution and stability arising from a different level of steric congestion. Preliminary studies suggest further that 5 a/H2 as well as 6 a enable the photo-induced hydrogenation of styrene to ethyl benzene, even if the mechanism and possibly catalytic nature of this process remain yet unknown.